Hitherto, an image formation method has been carried out in a conventional photographic plate making, which comprises bringing a light sensitive planographic printing plate material into contact with an original with an image, and exposing to light through the original. Recently, a computer to plate (hereinafter referred as to CTP) system has spread which comprises recording directly a digital image onto a light sensitive planographic printing plate material employing a laser. However, in order to carry out scanning exposure in a short time employing a laser, not only a laser with high output power but a light sensitive planographic printing plate material for CTP having sensitivity of 1,000 to 10,000 times the sensitivity of a conventional contact exposure type light sensitive planographic printing plate material are required. A planographic printing plate material for CTP with such a high sensitivity is expensive, and has problem in that sensitivity varies after long term storage.
A method is proposed in which digital signals are recorded on a light sensitive planographic printing plate material employing a digital mirror device (hereinafter referred to also as DMD) and an ultraviolet lamp as a light source. This method is advantageous in view of cost reduction, since it is possible to use a conventional contact exposure type light sensitive planographic printing plate material having sensitivity in ultraviolet regions. However, this method has problem that exposure time takes several times that in a conventional contact exposure. Thus, a light sensitive planographic printing plate material is required which has high sensitivity in the ultraviolet regions.
In Japanese Patent O.P.I. Publication No. 2004-29296 is disclosed a method of preparing a printing plate from a planographic printing plate material comprising a hydrophilic support and an image formation layer containing a polymerizable ethylenically unsaturated compound, a photopolymerization initiator, and a sensitizing dye having a specific structure, the method comprising the step of exposing the planographic printing plate material to ultraviolet rays corresponding to image data employing a DMD to polymerize the polymerizable ethylenically unsaturated compound. A printing plate prepared according to this method provides high printing durability, but has problem in printing durability. In Japanese Patent O.P.I. Publication No. 2004-20623 is disclosed a method of preparing a printing plate from a planographic printing plate material comprising a curable resin, the method comprising the step of exposing the planographic printing plate material to ultraviolet rays corresponding to image data employing a DMD, while heating, wherein curable resin at exposed portions are cured. This method provides a printing plate high printing durability, but has problem in that a heating system for heating the planographic printing plate material is complicate and impracticable, and the heating temperature is difficult to control, resulting in instability of image forming conditions. Further, a printing plate prepared according to this method has poor chemical resistance, and has problem in that when in a printing process ink stains at the non-image portions are removed employing a cleaning solution, so-called a plate cleaner, image portions, particularly small dots are likely to disappear.